Ambush is a special ability available to Generals and patriots. It costs 1000 Craft Craft to use and renders all units within the general's effective radius invisible to enemies. They will only attack when directed, causing them to break their stealth and suddenly appear out of nowhere from the enemy's perspective. Ambush is rendered ineffective by units and buildings that reveal hidden units, such as Scouts and lookouts. Ambush does not work on units that have recently attacked (within the last 33 seconds, or 500 frames), making it useful for sneaking up on an enemy, but not for running away from battle.
Ambush Effect

A general using ambush to move troops undetected.

While ambush is active, a yellow circle will appear around the general or patriot, indicating the area of effect. The circle is initially thick, but will gradually thin out and fade as the time remaining decreases.

The duration of ambush can be increased by researching the Fort's Strategy branch of technologies, starting at 30 seconds initially to 75 seconds when fully upgraded.